With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Leaf seals are formed from sections of leaf material appropriately presented in order to create a seal surface from juxtaposed leaf edges of respective leaves in an assembly. Typically the leaves are arranged circumferentially about a rotating shaft in order to present the leaf edges and therefore the seal surface towards that shaft in order to provide a seal barrier. Typically, spacer members are provided between each leaf in order to correctly arrange the seal elements for presentation of the leaf edges and therefore the seal surface. These spacers may be independent components or integrally formed with each leaf. The leaf edges and so the seal surface effectively floats upwards and downwards relative to a rotating surface.
In a gas turbine engine, leaf seals may be used to form a seal between a static component and a rotating component, between two relatively rotating components, or even between two static components in order to maintain a relatively high pressure on one side of the seal and relatively low pressure on the other. FIG. 2, which shows schematically, for example, a cut-away perspective view of a portion of a leaf seal assembly 31 comprises a pack of leaves 32 extending from spacers 33 secured in a housing comprising a backing ring 34 with coverplates 35. The leaves 32 present leaf edges 36 towards a surface 37 of a rotating component generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the leaf edges 36 of the leaves 32 act against the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is generally compliant in order to adjust with rotation of the surface 11 to ensure that a good sealing effect is created.
FIG. 3 shows schematically a more detailed view of part of the leaf pack of the assembly 31. The spacers 33 are present between the root portions of all the leaves 32 and are generally required in order to ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them. By varying the thickness of the spacers 33, the leaf thickness, the leaf length and the angle at which the leaves are positioned, it is possible to change the geometry of the leaf pack. For example, as can be seen in FIG. 4, by changing the relative thicknesses of spacers 33 and the leaves 32, the seal pack can be altered from a non-contact to a contact (arrowed) type pack with contact adjacent to the leaf edges 36. The spacers 33 may be integral with the leaves, or may be separate parts.
Vibration of the leaves, caused by the flow through the seal can be a problem as it may cause fatigue failure in the leaf material. Such cracks tend to grow quickly, leading to loss of leaf material.
The above problem occurs both in leaf packs with straight leaves and in leaf packs within which inter-leaf contact exists at the leaf tips. Large leaf-to leaf interference at the tips has been seen to alleviate this problem. A similar effect has also been seen with very thick leaves.
In large diameter seals, it is not possible to create significant amount of leaf-to-leaf interference at the leaf tips. This may lead to a limitation of the leaf seal to smaller diameter applications